Limited MHC class I intron 2 repertoire variation in bonobos

Common chimpanzees (Pan troglodytes) experienced a selective sweep, probably caused by a SIV-like virus, which targeted their MHC class I repertoire. Based on MHC class I intron 2 data analyses, this selective sweep took place about 2-3 million years ago. As a consequence, common chimpanzees have a skewed MHC class I repertoire that is enriched for allotypes that are able to recognise conserved regions of the SIV proteome. The bonobo (Pan paniscus) shared an ancestor with common chimpanzees approximately 1.5 to 2 million years ago. To investigate whether the signature of this selective sweep is also detectable in bonobos, the MHC class I gene repertoire of two bonobo panels comprising in total 29 animals was investigated by Sanger sequencing. We identified 14 Papa-A, 20 Papa-B and 11 Papa-C alleles, of which eight, five and eight alleles, respectively, have not been reported previously. Within this pool of MHC class I variation, we recovered only 2 Papa-A, 3 Papa-B and 6 Papa-C intron 2 sequences. As compared to humans, bonobos appear to have an even more diminished MHC class I intron 2 lineage repertoire than common chimpanzees. This supports the notion that the selective sweep may have predated the speciation of common chimpanzees and bonobos. The further reduction of the MHC class I intron 2 lineage repertoire observed in bonobos as compared to the common chimpanzee may be explained by a founding effect or other subsequent selective processes.